AIP and breast cancer: In addition, the expression of another ERα-regulate gene GREB1 (growth regulation by estrogen in breast cancer 1) [38] is also increased upon XAP2 depletion in MCF-7 cells (Fig. 1B, compare Scr E2 and hsiXAP2 E2), suggesting a suppressive effect of XAP2 on the expression of ERα target genes.